One of the most critical factors in painting is drying time. To accelerate the drying process, heat from a thermal oven is frequently used. However, commercial ovens are expensive to buy and operate, and the physical properties of the painted materials can make oven drying non-viable. For example, plastic is used to form many industrial products, but heat distorts and weakens plastic. Higher temperatures also pose a problem with timber and fiber cement, which contain a high percentage of moisture. Heating these materials causes water to leave the substrate, leading to warping, splitting and other forms of stability loss. Other materials, such as glass and metal, which are unaffected by heat, have high specific heat capacities and therefore require a great deal of energy in order to maintain the surface at an appropriate temperature for drying. Additionally, in order to avoid breakage of glass, uniform heating and cooling are critical. In light of such problems, manufacturers frequently dry products by setting them aside between coats. Not only do lengthy curing times slow deliveries and require valuable space, they also increase the chance that inclusions will contaminate the wet film.